Can drive the 1988 XJS for 10 miles or so, then the temp guage will hover right above the "N" if at a stop light i can cut the car off and crank it before the light turns green and the needle will sit under the "N" which i love for several miles before rising above the dreaded "N" once again. Sometimes the same trip will result in not running hot at all........any ideas ??????

 

Answered your post in the General Section.

Had no idea there was a "STILL" in the complaint.

May be the temp gauge itself, either the sender unit, or the well documented dash cluster issues.

Fitting a mechanical gauge using the same hole as the Jag sender unit will give you a more accurate reading of just what is going on.

Then the hunt can begin.

 

Guess ill give it whurl, u feel as if the threading will not be an issue ?

 

gregg, my 1989, sometimes the needle shoots up to the top of the gauge, as I sit there nonchalantly NOT WORRYING, most times it stays just below N. I used to freak out when it shot up, and, stayed there, then a few miles later, came down, rather quickly. Two mechanics, whom I respect, both told me "It is mechanically impossible" for the engine to heat up, or cool off, so quickly. Also, I got an infrared temperature gun, and, went over the entire engine when the needle was pegged. No high temps. A couple times. Using the gun, I found a problem in my '98 Buick, the fans weren't coming on at 220 degrees, so it can be a good diagnosis tool. Anyway, I now disregard the quick movements of the gauge. Oh, I had the coolant and both stats changed, and, have some water wetter in there.

 

The threads may require some thinking.

I used one of the threaded holes on mine at the rear of the RH water rail, coz we got no senders in that rail, just plugs, but you guys got all sorts of wierd stuff.

Floating temps CAN, sometimes, be the pressure cap on the LH metal header tank NOT holding correct pressure. This I know for sure. My NEW 15psi cap was blowing off at 6psi, and when I eventually found that, the next cap fixed all the oddness in that V12.

I took my cap tester to the parts store and went through all his caps until I got one that I liked. NOT cheap rubbish, CPC brand, and they have made caps forever.

 

My wife's Van will do that.... when the coolant level is low... the temp sensor is exposed to steam instead of water at those times of excessive gauge swings...


finding the leak and fixing it and filling it up solved it for me...

 

To me it sounds like an electrical problem. Actual high temp around the sensor should give some indication near it using the infrared thermometer. Most temp sensors and I believe the XJS is the same give full high temperature reading when the wire form the gauge is grounded (0 ohms) and shows cold when the circuit is open. When the engine temp gauge is showing hot try disconnecting the wire to the sensor. It should go to cold. If not there is a short somewhere in the wiring.

 

I agree that, as described, this is likely an electrical problem.

However, I'll toss out that, in my experience, air pockets in the cooling system can cause crazy gauge fluctuations. Has the cooling system been opened up recently....and perhaps not properly bled?

Cheers
DD

 

Thats what I was thinking. Air pockets lead to "steam readings"

 

Swiss Cheese....
letting steam out and air in...
in the first picture you will notice a lot of rust... ? and the pipe going to the banjo bolt a little discolored at the rubber hose joint...
upon removing all that to scrape and paint I found that the ends of the piping for the top of the radiator were Swiss cheesed with holes just under the rubber on the free side of the hose clamp letting coolant steam and air go either way... steam out, air in... all three ends of the pipe were damaged in this manner... along with 3 pin holes in the top radiator tube... ( only found with judicious filling of the radiator and driving it to temp...) said problems have been resolved... but I found the pin holes in the radiator, not the holes in the pipe until I took everything apart... High temp water proof epoxy paste to repair pipes... so far so good... that epoxy is good for all sorts of high pressure High temp applications...

 

How she ran before... (filled her before every trip) (about 1.5 gallons /8 miles trip) and
the repairs new radiator fixed pipes...
How she ran after filled once burped and here we are...

 

My guess is a combination of failing fan clutch and air in system.

 

Consider that the issue could be corroded connections at the back of the instrument panel. I don't have an 88, but an 87. How much they changed the wiring connections to the instruments I've no fact on, but a corroded or loose connection to the temp gauge could be part of the issue.

You'll be shocked (hopefully) when you see how flimsy the connections are on the back of the instrument panel. All par for the course :-)

John
1987 XJS V12